(a) Field of the Invention
The present invention relates to an in-plane switching scheme liquid crystal display (LCD) unit and, more particularly, to an in-plane switching scheme LCD unit having driving electric field parallel to substrates and achieving a wide view angle while suppressing a color change.
(b) Description of the Related Art
LCD units have advantages of smaller thickness, lower weight and lower power dissipation. Among other LCD units, an active matrix LCD unit wherein each of pixels arranged in a matrix is driven by an active element, such as a thin film transistor (TFT), is expected for use as a high-performance flat panel display unit.
A conventional active matrix LCD unit (AM-LCD) generally includes a twisted-nematic liquid crystal (TN-LC) layer and takes advantage of the electric-optical effect thereof by sandwiching the LC layer between a pair of substrates and applying the LC layer with an electric field substantially perpendicular to the surfaces of the substrates for operation of the LC layer.
U.S. Pat No. 3,807,831 discloses an AM-LCD unit using an in-plane switching scheme wherein the LC layer is operated by a lateral (in-plane) electric field parallel to the substrates. The disclosed LCD unit includes a pair of comb-shape electrodes, with the teeth of both the electrodes being alternately arranged.
Patent Publication JP-B-63-21907 discloses an AM-LCD unit which takes advantage of the electric-optical effect of the TN-LC layer. The disclosed LCD unit also includes a pair of comb-shape electrodes by which parasitic capacitance is reduced between the common electrode and the drain bus lines and between the common electrode and the gate bus lines.
FIG. 1 shows the AM-LCD unit using an in-plane switching scheme, referred to as an IPS-LCD unit hereinafter. The IPS-LCD unit includes a pair of front and rear glass substrates 11 and 12 sandwiching therebetween a LC layer 20, wherein the second substrate 12 mounts thereon a pair of comb-shape electrodes 70. By applying a driving voltage between the comb-shape electrodes 70, a lateral electric field is generated in the direction perpendicular to the extending direction of the teeth of the comb-shape electrodes 70 and parallel to the surfaces of the substrates 11 and 12. The lateral electric field rotates the orientation of the LC molecules, whereby the transmittance of the LC layer 20 is controlled in each pixel.
In the IPS-LCD unit of FIG. 1, the orientations of the LC molecules in each pixel should be determined in a specified direction by the application of the drive voltage for a stable and uniform image of the pixel. This is generally achieved by the configuration of the initial orientation xcfx86LC0 of the LC molecules, which is somewhat deviated from the direction perpendicular to the direction of the lateral electric field. In other words, the LC layer 20 is subjected to an initial orientation alignment so that the initial orientation xcfx86LC0 of the LC molecules is somewhat smaller than 90xc2x0 from the extending direction of the teeth of the comb-shape electrodes 70.
In the description to follow, angle xcfx86 of the direction of the electric field or orientation of the LC molecules is defined from the direction perpendicular to the extending direction of the teeth, with the counter-clockwise rotation as viewed from the front substrate being the positive. The initial orientation xcfx86LC0 of the LC molecules are generally determined as 45xc2x0xe2x89xa6xcfx86LC0xe2x89xa690xc2x0 for assuring a sufficient contrast while rotating the LC molecules by more than 45xc2x0. In the illustrated configuration of FIG. 1, the LC molecules are rotated by the driving electric field E1 in the clockwise direction as viewed from the front substrate or first substrate 11, as shown by the solid arrow, due to the initial orientation being somewhat deviated in the clockwise direction from the extending direction of the teeth of the electrodes 70.
If the LCD unit of FIG. 1 is sandwiched between a pair of polarizing plates having orthogonal polarization axes, the light transmittance T upon application of a driving voltage is expressed by the following equation:
T=xc2xdxc3x97sin2{2(xcfx86Pxe2x88x92xcfx86LC)}sin2(xcfx80xc2x7xcex94nxc2x7d/xcex)xe2x80x83xe2x80x83(1)
wherein xcfx86LC, xcfx86P, xcex94n, d, and xcex are orientation of the LC molecules upon application of the driving voltage, orientation of the passing axis of the polarizing plate disposed on the incident side of the back-light, the birefringence anisotropy of the LC layer, the cell thickness or the thickness of the LC layer, and the wavelength of the back-light, respectively. The orientation xcfx86A of the passing axis of the polarizing plate disposed on the light emitting side is expressed by:
xcfx86A=xcfx86P+90xc2x0 or xcfx86A=xcfx86Pxe2x88x9290xc2x0.
According to equation (1), the transmittance T is controlled by using a driving electric field parallel to the substrates to change the orientation of the LC molecules. If the orientation of the passing axis of one of the polarizing plates is aligned with the orientation of the initial orientation of the LC molecules, i.e., xcfx86LC0=xcfx86P or xcfx86LC0=xcfx86A, the LCD layer assumes a dark state upon application of no driving voltage whereas assumes a bright state upon application of the driving voltage. In the latter state, when the orientation of the LC molecules are rotated by 45xc2x0 by the driving electric field, the LCD unit assumes a brightest state thereof due to a maximum transmittance thereof. Alternatively, a configuration may be employed by changing the arrangement of the polarizing plates so that the LC layer assumes a dark state upon application of the driving voltage.
In the above description, it is stated that all the LC molecules sandwiched between the substrates are rotated by a uniform angle, for simplification of the description. In the display unit using the birefringence or double refraction, the light having a wavelength satisfying the relationship xcex94nxc2x7d=xcex/2 can pass the LC layer most efficiently. Thus, in order to obtain an excellent a white color image or a multi-color image by using a color filter, the birefringence anisotropy xcex94n and the thickness xe2x80x9cdxe2x80x9d of the LC layer are typically controlled so that the central wavelength of the spectrum of the transmitted light is set at 550 nm, i.e., xcex94nxc2x7d=275 nm. In a practical LCD unit, since the LC molecules disposed at the boundary between the LCD layer and the substrate is relatively firmly fixed to the substrate to assume less rotation, it is preferable that the birefringence anisotropy xcex94n and the thickness xe2x80x9cdxe2x80x9d be designed so that birefringence xcex94nxc2x7d resides between 280 and 330 nm.
WO91/10936 describes improvement of the view angle characteristics in the IPS-LCD unit using a TN-LC layer, and the IPS-LCD unit having such improved characteristics is expected for use as a large screen monitor.
FIG. 2 shows the view angle dependency of the relationship between the driving voltage and the light transmittance in the improved IPS-LCD unit. The view angles include an azimuth view angle xcfx86obs which is defined by a view direction of the observer measured with respect to the direction perpendicular to the extending direction of the teeth, and a polar view angle xcex8obs which is defined by an angle with respect to the perpendicular to the substrates. In FIG. 2, curve (I) shows view angles of xcex8obs=0 and xcfx86obs=0, and curve II shows xcfx86obs=40xc2x0, curve III shows xcfx86obs=85xc2x0, curve IV shows xcfx86obs=xe2x88x9250xc2x0 and curve V shows xcfx86obs=xe2x88x925xc2x0, with the polar view angle xcex8obs unchanged.
In the graph, the sample of the LC cell used for measurements has a configuration wherein xcfx86LC0=85xc2x0, xcfx86P=85xc2x0 and xcfx86A=xe2x88x925xc2x0. The pair of electrodes are of comb-shape, wherein the width of the teeth is 5 xcexcm and the distance between adjacent teeth is 15 xcexcm. In addition, the birefringence anisotropy xcex94n of the LC layer is 0.067 and the cell thickness xe2x80x9cdxe2x80x9d is 4.9 xcexcm. The conventional IPS-LCD unit has the advantage of relatively excellent view angle characteristics wherein the view angle dependency of the voltage-transmittance characteristics is low as shown by curves I to curve V.
The conventional IPS-LCD unit has, as described above, excellent view angle characteristics compared with the LCD unit having a TN-LCD layer and using a longitudinal electric field with respect to the absence of inversion in the gray-scale level. However, the conventional IPS-LCD has a disadvantage in that the observed color changes toward blue or red color depending on the view angle.
FIG. 3 shows the spectrum of the transmitted light from the LC cell of an IPS-LCD during assuming a bright state, wherein the sample of the LC cell measured herein was the same as the LC cell having the characteristics shown in FIG. 2. In FIG. 3, the spectrum of the transmitted light shown by curves (I) to (V) was measured at various azimuth view angles xcfx86obs of 0xc2x0, 40xc2x0, 85xc2x0, xe2x88x9250xc2x0 and xe2x88x925xc2x0, with the polar view angle xcex8obs=50xc2x0 being unchanged in the LC molecules, which had an initial orientation of xcfx86LC0=85xc2x0 and were applied with a driving voltage for a bright state.
The curve (I) in FIG. 3 shows that the orientation of the LC molecules is changed by the applied voltage to an orientation xcfx86LC=40xc2x0 from the initial orientation xcfx86LC0=85xc2x0, due to the rotation of the orientation of the LC molecules by 45xc2x0. As shown by the curve (II) in FIG. 3, the central wavelength of the transmitted light deviates at the view angle of xcfx86obs=40xc2x0 to a shorter wavelength from the curve (I), thereby having a tinge of blue color. On the other hand, as shown by the curve (IV) in FIG. 3, the central wavelength of the transmitted light deviates toward a longer wavelength at the view angle of xcfx86obs=xe2x88x9250xc2x0, thereby having a tinge of red color. In addition, similar tendency was observed at the view angles xe2x88x9250xc2x0 and xe2x88x925xc2x0 opposite to the view angles as specified above.
In the LC cell of the IPS-LCD unit, as described before, since the spectrum of the transmitted light depends on the birefringence xcex94nxc2x7d of the LC layer, the color change shown in FIG. 3 results from the view angle dependency of the apparent birefringence of the LC layer as detailed below.
If light is incident diagonally onto the LC cell as described above, the effective birefringence anisotropy xcex94nxe2x80x2 is expressed by:                               Δ          ⁢                      xe2x80x83                    ⁢                      n            xe2x80x2                          =                                                            n                e                            ⁢                              n                o                                                                                                          n                    e                    2                                    ⁢                                      cos                    2                                    ⁢                                      θ                    2                                                  +                                                      n                    o                    2                                    ⁢                                      sin                    2                                    ⁢                                      θ                    2                                                                                -                      n            o                                              (        2        )            
wherein xcex82, no, and ne are the angle between the longer axis of the LC molecules and the travelling direction of light, the refractive index of the LC layer against the ordinary ray which oscillates or polarizes in the direction perpendicular to the longer axes (optical axes) of the LC molecules, and the refractive index of the LC layer against lo extraordinary light oscillating or polarizing in parallel to the longer axes of the LC molecules, respectively.
FIGS. 4A and 4B show the view angle dependency of the birefringence index of the LC molecules of the IPS-LCD unit. In the case of perpendicular incidence, where xcex8=90xc2x0, the apparent birefringence anisotropy xcex94nxe2x80x2 is generally expressed by xcex94nxe2x80x2=nexe2x88x92no. At the view angle wherein the color deviates toward blue color, as shown in FIG. 4A, since the view angle is inclined toward the longer axes of the LC molecules, xcex82 less than 90xc2x0 and thus xcex94nxe2x80x2 is lower. On the other hand, at the view angle where the color deviates toward red color, as shown in FIG. 4B, since the view angle is inclined toward shorter axes of the LC molecules, xcex82=90xc2x0 and thus xcex94nxe2x80x2=xcex94n.
In the case of diagonal incidence, since the substantial thickness dxe2x80x2 of the LC layer is expressed by dxe2x80x2=d/cos xcex8obs, the substantial thickness dxe2x80x2 is larger irrespective of the direction of the rotation of the view angle. Both the changes in the birefringence anisotropy and in the thickness of the LC layer cause the change in the birefringence xcex94nxc2x7d, which in turn generates color change due to the view angle.
The above description will be summarized in Table 1 as follows.
JP-A-9-258369 describes improvement of the view angle characteristics of the LCD unit, which includes LC molecules having a uniform initial orientation and opposite directions in rotation of the orientation in each LC cell. FIG. 5 shows the LC cell described in the publication, wherein the LC molecules have a uniform initial orientation in the direction C6 and the pixel electrode C2 and the common electrode C3 have bends at the central region of each cell as viewed in the vertical direction. Thus, the pixel has two different directions of the electric fields C7, which rotate the LC molecules C8 in two directions. This configuration prevents inversion of gray scale levels as well as color change in the LC cell, because the two directions cancel each other in the color change.
The effective pixel area in each cell is generally defined by the total pixel area minus the area for the opaque electrodes. In the LC cell having the bends in electrodes, the bends cause reduction of ratio of the effective pixel area to the total pixel area of each cell and also may cause defects in each pixel such as an open-circuit failure.
In view of the above, it is an object of the present invention to provide an IPS-LCD unit capable of suppressing the color change depending on the view angle, inversion of gray scale levels, reduction of the ratio of the effective pixel area to the total pixel area, and defects in each pixel such as an open-circuit failure.
The present invention provides an IPS-LCD unit including first and second transparent substrates, a liquid crystal (LC) layer sandwiched therebetween to define a plurality of pixel areas, the LC layer including LC molecules having a substantially uniform initial orientation in a first direction, a pixel electrode and a common electrode disposed for each of the pixel areas for rotating the LC molecules, each of the pixel electrode and the common electrode including a plurality of first stripes extending substantially parallel to the first direction and a plurality of second stripes extending substantially perpendicular to the first direction, to define a plurality of zones in each of the pixel areas by one of the first stripes of the pixel electrode, one of the first stripes of the common electrode, one of the second stripes of the pixel electrode and one of the second stripes of the common electrode.
In accordance with the IPS-LCD unit of the present invention, the orientations of the LC molecules are rotated by the electric field generated between the pixel electrode and the common electrode in opposite directions zone by zone. Thus, the color change depending on the view angle can be cancelled by the orientations of the LC molecules in each pixel, thereby achieving excellent view angle characteristics substantially without a color change or an open-circuit failure.